A diagnostic ultrasound system transmits an ultrasonic wave from an ultrasound transducer into a subject, receives a reflected echo signal, which is ultrasonic wave, corresponding to the structure of the body tissue from inside the subject, and displays a cross-sectional image, such as a B-mode image, for diagnosis.
Recently, it has been proposed to measure ultrasound image data by apply a compression force to a subject according to a manual or mechanical method, determining the displacement in regions of the body caused by the compression on the basis of two sets of ultrasound image data measured at different times, and generating an elastic image representing the hardness or softness of the body tissue on basis of displacement data of the regions of the body. Accordingly, pressure sensors are provided on the back of a transducer element unit of an ultrasound transducer, the pressure applied to the ultrasound transducer by compressing the subject is determined, and an elastic image is displayed after determining Young's modulus. When the pressure exceeds predetermined threshold value of pressure, a light-emitting diode provided on the probe is illuminated. Such a measurement method is described in Patent Document JP2003-225239A.
However, according to this patent document, only Young's modulus is calculated by determining the pressure applied to the ultrasound transducer, and there is no mentioning of displaying compression state information on a screen.
It has been reported that the hardness of body tissue is non-linear and that the hardness of body tissue changes depending on the compression condition at the time the body tissue is compressed (for example, Krouskop T A, et al. Elastic Moduli of Breast and Prostate Tissue Under Compression. Ultrasonic Imaging. 1998; 20:260-274). Here, the compression condition include the change over time in the pressure applied to body tissue, the change in the compressed amount (the compressed amount of body tissue from a non-compressed state), and the compression speed.
In other words, since the hardness of body tissue changes depending on the compression condition, the measured elastic image also changes depending on the compression condition. This will be described with reference to FIGS. 1(A) to 1(C). FIG. 1(A) shows an example image of when compression is adequate, where the region of hard tissue is represented by a black circle, and other regions of soft tissue are represented in white. FIG. 1(B) shows an example image of when compression is excessive, where distortion is generated in the black circle representing the region of hard tissue, the border of the black circle and the regions of soft tissue in the periphery is unclear, and the contrast of the image is reduced. FIG. 1(C) shows an example image of when compression is inadequate. Since sufficient stress is not applied to the body tissue, points of zero distortion (areas that are recognized as being hard) are scattered through out the region that is uniformly soft, and the image becomes non-uniform.
However, conventionally, it has been difficult for an examiner to objectively determine whether elastic information recognizable from an elastic image of a region of interest differed depending on the compression condition because there has not been any consideration given to detecting the compression condition and displaying information on the compression condition in association with the elastic image. As a result, since the examiner is forced to carry out a diagnosis on the basis of an elastic image measured under a compression condition (adequate compression, inadequate compression, or excessive compression) based on subjectivity, it is disadvantageous in that the diagnostic result differs depending on the examiner's experience and proficiency.